1. Field of the Invention
The present invention relates to an image display device in which an electron beam is emitted from an electron emission element to a phosphor screen to display an image.
2. Description of the Related Art
In recent years, flat-panel image display devices have been developed as next-generation image display devices, in which a large number of electron emission elements oppose a phosphor screen. Although there are various types of electron emission elements, they basically utilize field emission. Display devices employing electron emission elements are generally called field emission displays (hereinafter referred to as “FEDs”). Among the FEDs, display devices using surface-conduction type electron emission elements are also called surface-conduction type electron emission displays (hereinafter referred to as “SEDs”). In this specification, the term “FED” is used as a collective term including SEDs.
Each FED has front and rear plates opposing each other with a narrow gap of about 1-2 mm, the peripheries of the plates being coupled to each other by a rectangular frame serving as side walls, thereby forming an evacuated envelope. The interior of the evacuated envelope is kept in a highly evacuate state of about 10−4 Pa. Further, a plurality of spacers are provided between the front and rear plates to support the plates on which the atmospheric pressure exerted.
A phosphor screen including red, blue and green phosphor film segments is formed on the inner surface of the front plate, while a large number of electron emission elements for emitting electron beams to activate the phosphor screen to emit light are provided on the inner surface of the rear plate. Further, a large number of scanning lines and signal lines are formed in a matrix and connected to the electron emission elements. An anode voltage is applied to the phosphor screen. When electron beams emitted from the electron emission elements are accelerated by the anode voltage and applied to the phosphor screen, the phosphor screen emits light to display thereon an image.
To obtain practical display characteristics in the FED constructed as above, it is necessary to use a phosphor screen similar to a standard cathode ray tube, and to form, on the phosphor screen, an aluminum thin film called a metal back film. In this case, it is desirable that the anode voltage applied to the phosphor screen be set to several kV, at least, and, if possible, to 10 kV or more.
However, the gap between the front and rear plates cannot be set so large in view of the resolution or the characteristics of the spacers, and need be set to about 1 to 2 mm. Accordingly, in FEDs, a strong electric field inevitably occurs in the small gap between the front and rear plates, which means that discharge may occur between the plates.
If no countermeasures are taken to suppress damage due to discharge, destruction or degradation of electronic emission elements, phosphor screen, driver IC discharge and driving circuits may well occur. These destruction and degradation, etc., will hereinafter be referred to as “discharge damage.”Under the circumstances that will cause such damage, in order to put FEDs to practical use, it is required to absolutely prevent discharge from occurring, for a long time. However, this is very difficult to realize.
Therefore, it is important to take measurements for reducing a discharge current to a level that enables discharge damage to be avoided or minimized to an ignorable extent. As a technique for this, a technique of segmenting a metal-back film (generally, an anode) is known. Metal-back segmentation can be mainly classified into first-dimensional segmentation in which the metal back film is divided only along one axis to form metal film strips, and second-dimensional segmentation in which the metal back film is separated along two axes to form metal film islands. Second-dimensional segmentation can make discharge current smaller than first-dimensional segmentation. The present invention relates to second-dimensional segmentation, and hence a publicly known example concerning first-dimensional segmentation is not shown in this description. Concerning the basic structure of the latter, see Jpn. Pat. Appln. KOKAI Publication No. 10-326538. Second-dimensional segmentation is disclosed in Jpn. Pat. Appln. KOKAI Publications Nos. 10-326538, 2001-243893 and 2004-158232.
When a metal back film is segmented, it is necessary to secure a route for a beam current in order to suppress a reduction in brightness within an allowable range, and also necessary to prevent discharge due to a potential difference that occurs between the gaps of the separated metal back layer segments. Regarding this point, Jpn. Pat. Appln. KOKAI Publications Nos. 10-326538 and 2004-158232 disclose a structure in which resistance layers are interposed between separated metal back layer segments. Further, Jpn. Pat. Appln. KOKAI Publication No. 2001-243893 discloses a structure in which separated metal back layer segments are connected to a power supply line extending close to them via respective resistance layers. Jpn. Pat. Appln. KOKAI Publication No. 2000-251797 also discloses interposition of resistance layers between metal back layer segments, although it contains no embodiments related to second-dimensional segmentation.
In the configuration of a typical FED, R, G and B pixels are arranged in the X-axis. Further, in general, it is preferable that R, G and B pixels are arranged in a square or substantially square matrix. Accordingly, in second-dimensional division, the X-axial (horizontal) gap Gx of separated metal back layer segments is smaller than the (vertical) Y-axial gap Gy of the separated metal back layer segments.
In general, in second-dimensional segmentation, it is important to set the resistance Rx across the gap Gx and the resistance Ry across the gap Gy to respective preset values. It can be understood from Jpn. Pat. Appln. KOKAI Publications Nos. 10-326538, 2001-243893, 2004-158232 and 2000-251797 that conventionally, the resistance Rx is assumed to actually be adjusted by a resistance layer provided in the gap Gx. However, since the gap Gx is small, a highly accurate process is required to form such a structure, which is not desirable for mass production. Further, to minimize discharge current, it is desirable to maximize the resistance Rx. In this case, high voltage occurs at the gap Gx during discharge and hence discharge may occur at the gap Gx. To avoid this, it is desirable to maximize the gap Gx so as to increase the withstand voltage. However, when the resistance Rx is adjusted by a resistance layer provided in the gap Gx, it is also necessary to secure a contact area between each separated metal back layer segment and resistance layer. This is an obstacle to broaden the gap Gx.